A pressure-sensitive pointing device installed in a keyboard of a notebook PC, and so forth is made up such that when a user presses down an operation console of the device in a desired direction with a fingertip, a load applied in that direction is detected by a strain sensor built inside the device, and a detection signal of the strain sensor is processed, whereupon a pointer such as a cursor, or the like, shown on a display of the notebook PC, is caused to shift. At this point in time, a shifting direction of the pointer is determined correspondingly to a direction of the load applied to the tip of the device, and a shifting speed is determined correspondingly to magnitude of the load.
As a conventional signal processing system for processing output signals of a pressure-sensitive pointing device (hereinafter referred to also as a pointing device), there is available an input unit as disclosed in Patent Document 1. FIG. 6 is a block diagram showing the input unit.
An output signal of a pressure-sensitive pointing device 51 is inputted to such a signal processing system 41. The pressure-sensitive pointing device 51 comprises a strain sensor 51a for detecting a load in a plus direction along an x-axis (hereinafter referred to as a +X direction), a strain sensor 51b for detecting a load in a minus direction along the x-axis (hereinafter referred to as a −X direction), a strain sensor 51c for detecting a load in a plus direction along a y-axis (hereinafter referred to as a +Y direction), and a strain sensor 51d for detecting a load in a minus direction along the y-axis (hereinafter referred to as a −Y direction), the respective loads resulting from an operation of an operation console, not shown. The strain sensors 51a, 51b, 51c, 51d each are made up of a strain gauge such as a piezoelectric resistance element, and when the operation console, not shown, is operated in the +X direction, the −X direction, the +Y direction, and the −Y direction, respectively, the strain sensors 51a, 51b, 51c, 51d each are pressed downward corresponding to respective directions of operations, and respective resistance values thereof undergo a change due to the respective loads applied thereto. The strain sensors 51a, 51b are connected in series, and the strain sensors 51c, 51d are connected in series. In this case, the x-axis refers to an axis in a side-to-side direction or in the lateral direction of the pointing device 51, as seen from a user, while the y-axis refers to an axis in a front-to-back direction or the longitudinal direction of the pointing device 51. Further, the x-axis corresponds to a side-to-side direction, or the lateral direction, on a display of a notebook PC, and so forth, in which the pointing device 51 is installed, and the y-axis corresponds to a front-to-back direction or the longitudinal direction, on the display. Such series-connected circuits as described are connected in parallel, forming a parallel-connected circuit, and a power supply voltage Vdd is fed to the parallel-connected circuit.
The four strain sensors, under no load, are equal in resistance value, however, when the operation console is operated in the +X direction, the −X direction, the +Y direction, and the −Y direction, respectively, the resistance value of any of the strain sensors 51a, 51b, 51c, 51d, positioned in the direction of an operation, undergoes a change, whereupon a strain along the x-axis direction is detected as a voltage change via a node 51e between the strain sensors 51a, 51b while a strain along the y-axis direction is detected as a voltage change via a node 51f between the strain sensors 51c, 51d. If the operation console is pressed down in a slanting direction (a direction within a plane containing the x-axis, and the y-axis, but nonparallel to the x-axis and the y-axis) at this point in time, there are detected a stain corresponding to an x-axis direction component of a vector in a direction in which the operation console is pressed down, and a stain corresponding to a y-axis direction component of the vector. Upon removal of the load, the respective resistance values of the strain sensors revert to the respective resistance values thereof, under no load, and potentials at the nodes 51e, 51f, respectively, revert to respective values before the voltage change.
Low-pass filters 52, 53 comprise capacitors 52a, 53a, and resistors 52b, 53b, respectively, and with the low-pass filters 52, 53, an upper cut-off frequency is set to on the order of 150 Hz so as to remove low frequency noise components out of respective output signals of operational amplifiers 43, 44, to be described later. Further, an output side of the low-pass filter 52 is connected to terminals 41a, 41b of the signal processing system 41, respectively, and an output side of the low-pass filter 53 is connected to terminals 41c, 41d of the signal processing system 41, respectively.
The signal processing system 41 comprises a digital processing circuit 42 having a CPU 42a, ROM 42b, RAM 42c, for executing control of the signal processing system 41 in whole, and so forth, the operational amplifier 43 having an inverting input side connected to the terminal 41a, and a noninverting input side connected to an output side of a digital-to-analog converter (hereinafter referred to as a DAC) 46 to be described later, an output side of the operational amplifier 43 being connected to the terminal 41b, the operational amplifier 44 having an inverting input side connected to the terminal 41c, and a noninverting input side connected to an output side of a DAC 47 to be described later, an output side of the operational amplifier 44 being connected to the terminal 41d, an analog switch SW 11 connected to the output side of the operational amplifier 43, an analog switch SW 12 connected to the output side of the operational amplifier 44, an analog-to-digital converter (hereinafter referred to as an ADC) 45 having an input side connected to a common output side of the analog switches SW 11, SW 12, and having an output side connected to an input side of the digital processing circuit 42, the DAC 46 having an input side connected to an output side of the digital processing circuit 42, and having the output side connected to the noninverting input side of the operational amplifier 43, and the DAC 47 having an input side connected to the output side of the digital processing circuit 42, and having the output side connected to the noninverting input side of the operational amplifier 44. The low-pass filters 52, 53 serve as feedback circuits of the operational amplifiers 43, 44, respectively.
There is described hereinafter an operation of the signal processing system 41 having such a configuration described as above.
The voltage corresponding to the strain along the x-axis direction, outputted from the node 51e of the pressure-sensitive pointing device 51, is inputted from the terminal 41a to the inverting input side of the operational amplifier 43. Similarly, the voltage corresponding to the strain along the y-axis direction, outputted from the node 51f of the pressure-sensitive pointing device 51, is inputted from the terminal 41c to the inverting input side of the operational amplifier 44. Reference data outputted from the digital processing circuit 42 is converted into an analog reference voltage by the DAC 46 to be subsequently inputted to the noninverting input side of the operational amplifier 43. The reference data outputted from the digital processing circuit 42 is converted into the analog reference voltage by the DAC 47 to be subsequently inputted to the noninverting input side of the operational amplifier 44. Now, assuming that the strain sensors 51a, 51b, 51c, 51d, under no load, each have a resistance value Rs, and the resistors 52b, 53b of the low-pass filters 52, 53, respectively, each have a resistance value Rf, the operational amplifiers 43, 44 each have a gain of −{Rf/(Rs/2)}, so that a change (on the order of ±10 mV) in the voltage corresponding to the strains along the x-axis, and the y-axis, respectively, can be amplified to a voltage change (on the order of ±1 V) centering around the analog reference voltage.
Rectangular waves Asw 11, and Asw 12, undergoing an alternate change in level for every detection period T1 (for example, every 10 msec) as shown in FIG. 7, are inputted as switching control signals to the analog switches SW 11, SW 12, respectively. The analog switches SW 11, SW 12 are turned ON, respectively, during a period when the rectangular waves Asw 11, and Asw 12 are being held high, respectively, while the analog switches SW 11, SW 12 are turned OFF, respectively, during a period when the rectangular waves Asw 11, and Asw 12 are being held low, respectively, so that the analog switches SW 11, SW 12 are alternately turned ON during the detection period T1. Accordingly, a voltage Vx 11, corresponding to the strain along the x-axis direction, and a voltage Vy 11, corresponding to the strain along the y-axis, alternately appear on the common output side of the analog switches SW 11, SW 12, that is, on the input side of the ADC 45, as shown in FIG. 7. Those voltages Vx 11, Vy 11, corresponding to the respective strains, are digitized by the ADC 45 before being inputted to the digital processing circuit 42.
Patent Document 1: JP 7-319617 A